May 26, 2026
Flash flooding is one of the most disruptive and dangerous weather hazards in cities, and new research focused on Greater Sydney shows that the conditions that trigger it are becoming more common.
The study, led by Hub researchers at UNSW and partner institutions, brings together high-resolution weather station observations and state-of-the-art climate modelling to look closely at very short-duration rainfall events, those lasting just minutes to an hour. These are the kinds of events that overwhelm drains, flood streets, and catch communities off-guard.
What makes this research new is its focus not just on how much rain falls, but how quickly it falls. The findings show a clear shift toward rainfall that is shorter, more intense, and more frequent, a powerful combination when it comes to flash flooding.
A double whammy for cities
The research shows that the most intense rainfall events are getting stronger, particularly over 5–10 minute periods. At the same time, these extreme events are also projected to happen more often, with the frequency of the heaviest hourly rain events potentially increasing by up to 125% by late this century under high greenhouse gas emission scenarios.
Yet there’s a twist. Despite heavier downpours, the total number of wet hours is projected to decline. This means rain in the future is likely to arrive in sharper bursts, rather than long, steady falls.
For cities like Sydney, this may be a recipe for flash flooding. Stormwater systems, roads, and buildings are typically designed using historical rainfall patterns that assume slower, more evenly distributed rain. Short, intense rainfall events test those assumptions.
Who needs to pay attention?
This shift in rainfall matters, including:
- Councils manage local roads, drains, parks, and public spaces that are all highly vulnerable to sudden surface flooding. Knowing that extreme rainfall is intensifying at minute-to-minute scales helps explain why streets can flood even when daily rainfall totals aren’t record-breaking.
- Water utilities manage stormwater and wastewater systems. Intense rain falling faster than systems can handle it increases the risk of overflows, infrastructure damage, and water quality issues.
- Engineers designing water sensitive infrastructure will need to account for these changes in rainfall patterns in the design of water drainage systems to ensure they perform their desired purpose for the life of the asset.
- Schools, hospitals, and public facilities are critical during emergencies. Flooded access points, damaged equipment, and disrupted services can have serious consequences, particularly when extreme rainfall occurs during busy periods.
- Homeowners and residents are also part of the picture. While individual actions can’t stop extreme rain, maintaining gutters and drains, improving on‑site drainage, and reducing hard surfaces can help limit local water build‑up during intense downpours.
Why this research matters now
By combining high-resolution observations with climate projections, this study shows that flash-flood-driving rainfall is not just a future problem, it is already intensifying. It also highlights an encouraging message: under strong emissions reductions, the worst changes in extreme rainfall could be limited later this century.
Understanding how rainfall is changing, not just rainfall totals, but in timing and intensity, is a crucial step toward managing risk. As Sydney continues to grow, awareness of these faster, fiercer rain events will be key to protecting infrastructure, services, and communities in a warming climate.
Similar studies are underway for capital cities across Australia.
To read the full journal paper (opens in new browser) >
To find out more about the Hub CS5.7 Extreme rainfall project >
